1. Field of the Invention
The present invention relates generally to prism assemblies used for measuring precise distances such as in surveying and more particularly, to an integrated prism assembly which is adjustable in elevation, has selectable offsets, provides a stationary target and a level and which can accommodate a plumb line for accurate positioning of the assembly with a minimum of inconvenience.
2. Prior Art
The use of prism assemblies in the surveying art as retroreflectors for precisely measuring distances is well-known. By way of example, see U.S. Pat. No. 4,343,550 which is assigned to the assignee of the present invention. Typically such prism assemblies tend to be relatively large, cumbersome structures utilizing one or more three inch diameter prisms, associated targets to permit easier sighting and complex brackets to permit elevation adjustment and offset variation in the field. However with technological improvements in laser optics and particularly in regard to the distance measurement capabilities of laser distance measuring instrumentation, there has been a recent trend in the industry toward the use of smaller prism assemblies which employ one inch prisms and a relatively miniaturized structure associated with such prisms to permit a greater degree of portability and convenience in the field. Such smaller and simpler prism assemblies typically can be carried in a coat pocket or small pouch that may be attached to a belt of the user thereby significantly increasing the convenience of making a large plurality of measurements at different locations in the field over the course of a work day. There is however a trade-off that one must make between convenience of portability and performance capabilities. More specifically, the more portable one wishes to make these miniaturized prism assemblies, the more difficult it becomes to provide certain performance characteristics such as vertical alignment positioning capability, ease of sighting from a long distance, leveling capability, ease of offset and elevation adjustment and the like. On the other hand, in general, the more these performance characteristics the prism assembly is designed to possess, the more difficult it becomes to provide miniaturization and portability and ease of use in the field.
FIGS. 1-8 of the accompanying drawings illustrate various configurations of prior art miniature retro-reflective prism assemblies of the type hereinabove discussed and reveal various ways in which there has been an attempt in the prior art miniature prism field to accommodate the desire for both portability and performance. Thus, for example some of the prior art configurations shown in FIGS. 1-8 of the accompanying drawings illustrate the capability of providing a plumb line and elevational control to position the prism at a desired location and direction for improving measurement capability. Other configurations of the prior art shown in the accompanying drawings illustrate the ability to change the offset of the prism such as between 0 and 30 millimeters and still other prior art configurations have a target to improve the sightability of the prism from a remote location where the laser instrumentation may be located.
Unfortunately, none of the aforementioned prior art disclosed herein and otherwise known to the applicants, provides a miniature prism assembly capable of each one of the aforementioned features in an integrated unit which can be strung for a plumb line without moving the reflective prism housing from the yoke, which allows the plumb line to be strung in the same manner for 0 and 30 millimeter offset and which allows stringing of the plumb line while a stationary target remains on the yoke. Perhaps most importantly, there is no known prior art portable prism assembly that uses a stationary target which is easily mounted and which does not present any inaccuracies in the distance measurement because the center line does not move when the prism is tilted. There has therefore been a long-felt need for a miniature prism assembly of the type generally described above, but which provides all such features in one combined unit and also overcomes the aforementioned deficiencies of the prior art.